It is known both after pulping stages and also before and between separate bleaching stages that it is necessary to rinse out impurities which have been dissolved out of the wood or fibrous material during cooking or bleaching and to wash the added chemicals and/or reaction products out of the chemical pulp, in order to avoid undesirable properties in the final chemical pulp and to avoid polluting the environment.
At the present time the following basic different types of chemical pulp washers are known:
1) Drum washers with vacuum filter, CB-filter and pro-feed washers. PA1 2) Belt washers with a filter belt or using the double wire press technique. PA1 3) Washing presses or press washers, in which the pulp is dewatered only to a certain level of dry content. PA1 4) Diffusers: these allow for constant consistency by displacement washing in a tower.
Since the incorporated wash water must be concentrated by evaporation, one is forced to obtain maximum volumes of the portions extracted from the wood during the cooking process and the chemicals which have been added with maximum attainable concentration, and with use of as small volumes of wash water as possible.
Drum washers, belt washers and wash presses have the drawback that the material is fed in at relatively low consistency (1-5 weight percent based on dry solids content). Not until after a preliminary dewatering procedure does the actual washing process begin. The outlay in terms of intermediate tanks and pumps is relatively great. Since conventional diffusion washers are subjected to longer durations of operation and higher temperatures, they can be operated with a pulp consistency of only 10 to 12 weight percent.
Bleaching installations for chemical pulp materials today run mostly in the medium consistency range (8 to 15 weight percent), and some stages, for example, using ozone, can be carried out in the high consistency range. At this time there is no washing system which on the one hand operates entirely in the medium consistency range while also attaining a high output consistency.